Hydrology & Water Resources Engineering Unit II PDF

MISSION VISION CORE VALUES CHRIST is a nurturing ground for an individual’s Excellence and Service Faith in God | Moral Uprightness holistic development to make effective contribution to L...

MISSION VISION CORE VALUES CHRIST is a nurturing ground for an individual’s Excellence and Service Faith in God | Moral Uprightness holistic development to make effective contribution to Love of Fellow Beings the society in a dynamic environment Social Responsibility | Pursuit of Excellence  CHRIST Deemed to be University Contents Precipitation Components of Precipitation Losses Evaporation Definition Theory Factors Affecting Measurement of Evaporation Excellence and Service  CHRIST Deemed to be University PRECIPITATION Excellence and Service  CHRIST Deemed to be University PRECIPITATION Once precipitation reaches ground surface, it gets converted into various components based on; Topography Land Use – Land Cover Local Weather Condition Excellence and Service  CHRIST Deemed to be University PRECIPITATION AND ITS COMPONENTS The Components of Precipitation are: 1. Interception 2. Depression Storage 3. Infiltration 4. Deep Percolation 5. Evaporation 6. Transpiration 7. Runoff Excellence and Service  CHRIST Deemed to be University PRECIPITATION COMPONENTS 1. Interception: Precipitation that does not reach the soil, but is instead intercepted by the leaves, branches of plants and the forest floor. It occurs in the canopy, and in the forest floor or litter layer. Excellence and Service  CHRIST Deemed to be University PRECIPITATION COMPONENTS 2. Depression Storage: Water that is trapped in the small depressions that are characteristic of any natural surface with no possibility for escape as runoff. Excellence and Service  CHRIST Deemed to be University PRECIPITATION COMPONENTS 3. Infiltration: When water is applied to the surface of a soil, a part of it seeps into the soil to replenish the soil moisture deficiency. This movement of water through the soil surface is known as infiltration Excellence and Service  CHRIST Deemed to be University PRECIPITATION COMPONENTS 4. Deep Percolation: Hydrologic process, where water moves downward from surface water to groundwater. This process usually occurs in the vadose zone below plant roots and, is often expressed as a flux to the water table surface. Excellence and Service  CHRIST Deemed to be University PRECIPITATION COMPONENTS 5. Evaporation: Evaporation includes all processes by which water returns to the atmosphere as water vapour. Excellence and Service  CHRIST Deemed to be University PRECIPITATION COMPONENTS 6. Transpiration: Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Excellence and Service  CHRIST Deemed to be University PRECIPITATION COMPONENTS 7. Runoff: Runoff means the draining or flowing off of precipitation from a catchment area through surface flows into a stream or river. Excellence and Service  CHRIST Deemed to be University LOSSES What is LOSS? Based on the purpose of study, some components of precipitation are considered as loss. For a Hydro-Geologist: - Groundwater Except Infiltration and Deep percolation other components Losses For a Surface Water Hydrologist: - Streamflow Except Runoff other components are considered as Losses Excellence and Service  CHRIST Deemed to be University PRECIPITATION – COMPONENTS & LOSSES In this UNIT we determine Runoff for a particular Storm: Interception Depression Storage Infiltration 3 Deep Percolation Evaporation 1 Transpiration 2 Runoff Excellence and Service  CHRIST Deemed to be University EVAPORATION It is the process in which water changes from liquid to gaseous state at the free surface, below the boiling point through the transfer of heat energy. Excellence and Service  CHRIST Deemed to be University THEORY OF EVAPORATION ✓ Intermolecular attractive forces decrease w.r.t constant motion of molecules of water with a wide range of instantaneous velocities ✓ With addition of heat, this range and average velocity, increases. ✓ When molecules possess sufficient KE, they leave the water surface. ✓ In the similar manner the surrounding atmosphere contains water molecules and some of them may penetrate the water surface. ✓ The net escape of water molecules form the liquid state to gaseous state constitutes evaporation. Excellence and Service  CHRIST Deemed to be University FACTORS AFFECTING EVAPORATION The Rate of Evaporation (EL) is influenced by ✓ Vapour Pressure ✓ Temperature ✓ Atmospheric Pressure ✓ Wind Speed ✓ Size of Water Body ✓ Quality of Water Excellence and Service  CHRIST Deemed to be University FACTORS AFFECTING EVAPORATION 1. VAPOUR PRESSURE Dalton’s law of Evaporation, 𝐸𝐿 ∝ 𝐸𝑤 − 𝐸𝑎 𝐸𝐿 = 𝑐 𝐸𝑤 − 𝐸𝑎 EL = Rate of Evaporation c = constant Ew = Saturation Vapour Pressure @ Water Temperature Ea = Actual Vapour Pressure in the air Excellence and Service  CHRIST Deemed to be University FACTORS AFFECTING EVAPORATION 2. TEMPERATURE, T The other factors remaining constant, 𝐸𝐿 ∝ 𝑇 Excellence and Service  CHRIST Deemed to be University FACTORS AFFECTING EVAPORATION 3. WIND SPEED, U It influences the EL upto a critical value. If the wind velocity is large enough to remove all water vapour from the zone of evaporation, increment in U does not further influence the EL. This limit of wind speed is known as critical value. Excellence and Service  CHRIST Deemed to be University FACTORS AFFECTING EVAPORATION 4. ATMOSPHERIC PRESSURE, PATM Other factors are constant, EL is inversely proportional to Patm Decrease in barometric pressure, as in High Altitudes, increases Evaporation Excellence and Service  CHRIST Deemed to be University FACTORS AFFECTING EVAPORATION 5. QUALITY OF WATER EL reduces with respect to pure water. When a solute is dissolved in water, the VP of the solution is less than that of pure water; hence causes reduction in the rate of evaporation. Under identical conditions, Evaporation from sea water is 2-3% less than fresh water. Excellence and Service  CHRIST Deemed to be University FACTORS AFFECTING EVAPORATION 6. SIZE OF WATER BODY Areal extent and depth of water body influences the EL. Deep water bodies have more Heat Storage than Shallow water bodies. A deep lake may store radiation energy received in summer and release it in winter, causing less evaporation in summer and more in winter – in comparison to a shallow lake exposed to similar conditions. Excellence and Service  CHRIST Deemed to be University FACTORS AFFECTING EVAPORATION The Rate of Evaporation (EL) is influenced by ✓ Vapour Pressure ✓ Temperature ✓ Atmospheric Pressure ✓ Wind Speed ✓ Size of Water Body ✓ Quality of Water Excellence and Service  CHRIST Deemed to be University ESTIMATION OF EVAPORATION The most important component of hydrologic cycle is evaporation. Design of water supply schemes for domestic/irrigation/industrial purposes is based on net available precipitation Especially in arid zones, it is much more important to adopt water conservation practices. Accurate estimation of evaporation leads to reliable estimation of other components of precipitation Excellence and Service  CHRIST Deemed to be University ESTIMATION OF EVAPORATION METHODS: Evaporimeters Class A evaporation Pan ISI Standard Pan Colorado Sunken Pan USGS Floating Pan Empirical equations Meyer’s Formula Rohwer’s Formula Analytical methods Water Balance Method Energy Balance Method Mass Transfer Method Excellence and Service  CHRIST Deemed to be University ESTIMATION OF EVAPORATION EVAPORIMETERS Evaporimeters are water containing pans which are exposed to atmosphere and are used to simulate the conditions of lake or large water body to find evaporation. Loss of water from the pan is measured at regular intervals. Other meteorological data like; Humidity Wind Movement Air and Water Temperature Precipitation are also noted along with pan evaporation at that location. Excellence and Service  CHRIST Deemed to be University EVAPORIMETERS Correction of Pan Evaporation Data: It is difficult to simulate the realistic conditions, hence correction to pan evaporation data is required to determine the lake evaporation. 𝐿𝑎𝑘𝑒 𝐸𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑖𝑜𝑛 = 𝐶𝑝 𝑃𝑎𝑛 𝐸𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑖𝑜𝑛 Density of Evaporation Stations: As per WMO Arid Zones: One in every 30, 000 Sq.km Humid temperate zones: One in every 50, 000 Sq.km Cold Regions : One in every 1, 00, 000 Sq.km In India, average density is, One in 15000 Sq.km (220 stations) Excellence and Service  CHRIST Deemed to be University EVAPORIMETERS – CLASS A PAN Pan made up of GI Sheet with a size of 1210 mm dia and 255 mm depth supported by square wooden platform of 150 mm depth above GL to accommodate the circulation of air below the pan. Avg Pan Coefficient (Cp ) = 0.7 Excellence and Service  CHRIST Deemed to be University EVAPORIMETERS – ISI STANDARD PAN/ MODIFIED CLASS A PAN Pan Dimensions: 1220 mm dia with 255 mm depth Make: Copper sheet of 0.9 mm, tinned inside and painted white outside Platform: 1225 x 1225 x 100 mm wooden Measurement: A fixed point gauge in a stilling well Wire mesh on top of pan: Hexagonal wire mesh of GI to maintain uniform temperature and to prevent birds form drinking of water from pan Excellence and Service  CHRIST Deemed to be University EVAPORIMETERS – ISI STANDARD PAN/ MODIFIED CLASS A PAN Calibrated Cylinder: To add water to pan or remove from pan Evaporation from this pan is 14 % less as compared to Unscreened pan. Avg Pan Coefficient (Cp ) = 0.8 Excellence and Service  CHRIST Deemed to be University EVAPORIMETERS – COLORADO SUNKEN PAN To simulate the real lake conditions this pan is buried into the ground. Water level is maintained up to ground level in the pan Excellence and Service  CHRIST Deemed to be University EVAPORIMETERS – COLORADO SUNKEN PAN Dimensions: 920 x 920 x 460 mm Make: Unpainted GI sheet Advantages: Radiation and Aerodynamic Conditions are similar to lake Disadvantages: Frequent clearing of grass from surroundings Expensive to install Difficult to detect Leaks Avg Pan Coefficient (Cp ) = 0.78 Excellence and Service  CHRIST Deemed to be University EVAPORIMETERS – USGS FLOATING PAN Suitable for simulation of conditions of large water bodies Pan is set afloat in lake with the help of drum floats on raft. Water level in the pan is kept at same level as in lake Excellence and Service  CHRIST Deemed to be University EVAPORIMETERS – USGS FLOATING PAN Dimensions: 900 x 900 x 450 mm Size of raft: 4.25 x 4.87 m Diagonal Baffles: To prevent Surging in pan due to waves Avg Pan Coefficient (Cp ) = 0.8 Excellence and Service  CHRIST Deemed to be University EVAPORIMETERS – PAN COEFFICIENTS CP Evaporation Pans are not exact models of Large Reservoirs; Differ in Heat-Storing Capacity and Heat Transfer from the Sides and Bottom Sunken Pan reduces this deficiency Height of Rim of Pan affects the Wind Action over the surface. It also casts a shadow of variable magnitude over the water surface Heat-Transfer characteristics of the pan material is different from that of the Reservoir. 𝐿𝑎𝑘𝑒 𝐸𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑖𝑜𝑛 = 𝐶𝑝 𝑃𝑎𝑛 𝐸𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑖𝑜𝑛 Excellence and Service  CHRIST Deemed to be University EVAPORIMETERS – VALUES OF CP 𝐿𝑎𝑘𝑒 𝐸𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑖𝑜𝑛 = 𝐶𝑝 𝑃𝑎𝑛 𝐸𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑖𝑜𝑛 Types of Pan Average Value Range Class A 0.70 0.60 - 0.80 ISI Pan (Modified Class A) 0.80 0.65 - 1.10 Colorado Sunken Pan 0.78 0.75 - 0.86 USGS Floating Pan 0.80 0.70 - 0.82 Excellence and Service  CHRIST Deemed to be University FACTORS AFFECTING EVAPORATION The Rate of Evaporation (EL) is influenced by ✓ Vapour Pressure ✓ Temperature ✓ Atmospheric Pressure ✓ Wind Speed ✓ Size of Water Body ✓ Quality of Water Excellence and Service  CHRIST Deemed to be University ESTIMATION OF EVAPORATION EMPIRICAL EQUATIONS Evaporation based on Dalton’s Law 𝐸𝐿 ∝ 𝑒𝑤 − 𝑒𝑎 𝐸𝐿 = 𝑐 𝑒𝑤 − 𝑒𝑎 EL = Rate of Evaporation c = constant ew = Saturation Vapour Pressure @ Water Temperature ea = Actual Vapour Pressure in the air Excellence and Service  CHRIST Deemed to be University ESTIMATION OF EVAPORATION EMPIRICAL EQUATIONS General Equation to calculate Evaporation 𝐸𝐿 = 𝐾 𝑓 𝑢 𝑒𝑤 − 𝑒𝑎 EL = Rate of Evaporation (mm/day) K = constant f(u) = Wind Speed Correction function ew = Saturation Vapour Pressure @ Water Surface Temperature in mm of Hg ea = Actual Vapour Pressure in the air @ Specified height in mm of Hg Excellence and Service  CHRIST Deemed to be University EMPIRICAL EQUATIONS MEYER’S FORMULA 𝑈9 𝐸𝐿 = 1 + 𝐾𝑀 𝑒𝑤 − 𝑒𝑎 16 KM = 0.36 for Large Deep Water Bodies 0.5 for Shallow Water Bodies U9 = Mean Monthly Wind Speed (in km/hr) measured 9m above Ground Level Excellence and Service  CHRIST Deemed to be University EMPIRICAL EQUATIONS ROHWER’S FORMULA 𝐸𝐿 = 0.771 1.465 − 0.000732 𝑃𝑎 0.44 + 0.733𝑈0.6 𝑒𝑤 − 𝑒𝑎 Pa = Atmospheric Pressure in mm of Hg U0.6 = Mean Wind Speed (in km/hr) measured 0.6m above Ground Level Excellence and Service  CHRIST Deemed to be University ESTIMATION OF EVAPORATION ANALYTICAL METHODS Excellence and Service  CHRIST Deemed to be University ANALYTICAL METHODS WATER BUDGET METHOD ෍ 𝐼 − ෍ 𝑂 = ∆𝑆 + 𝐸𝐿 𝐸𝐿 = ෍ 𝐼 − ෍ 𝑂 − ∆𝑆 𝐸𝐿 = 𝑃 + 𝑉𝑖𝑠 + 𝑉𝑖𝑔 − 𝑇 + 𝑉𝑜𝑠 + 𝑉𝑜𝑔 − ∆𝑆 Excellence and Service  CHRIST Deemed to be University ANALYTICAL METHODS ENERGY BUDGET METHOD 𝐻𝑛 = 𝐻𝑎 + 𝐻𝑒 + 𝐻𝑔 + 𝐻𝑠 + 𝐻𝑖 Where 𝐻𝑛 = 𝐻𝑐 1 − 𝑟 − 𝐻𝑏 , Net Heat Energy received by the water surface Excellence and Service  CHRIST Deemed to be University ANALYTICAL METHODS ENERGY BUDGET METHOD 𝐻𝑛 = 𝐻𝑎 + 𝐻𝑒 + 𝐻𝑔 + 𝐻𝑠 + 𝐻𝑖 H a = Sensible heat transfer to the air H b = Back raidiation from the water body H e = Heat energy used in evaporation process = LE L  = density of water L = latent heat of evaporation E L = Evaporation H g = Heat flux into the ground H s = Heat stored in the water body H i = net heat conducted out of the system by water flow (adveted energy) Excellence and Service  CHRIST Deemed to be University ANALYTICAL METHODS MASS TRANSFER METHOD This method is based on turbulent mass transfer in boundary layer to calculate the mass water vapour transfer from the surface to the surrounding atmosphere. Excellence and Service  CHRIST Deemed to be University ESTIMATION OF EVAPORATION LOSS IN RESERVOIRS Though the analytical methods give better results, pan evaporation data has greater significance as per practical considerations. The volume of evaporation loss from a reservoir in a month is calculated as 𝑉𝐸 = 𝐴𝐸𝑝𝑚 𝐶𝑝 VE = volume of water lost in evaporation in a month (m3) A = Average reservoir area during the month (m2) Epm = pan evaporation loss in a month (in m) = EL (mm/day) X No. of days in the Month X10-3 Cp= Relevant Pan Coefficient Excellence and Service  CHRIST Deemed to be University REDUCTION OF EVAPORATION IN RESERVOIRS 1. Reduction of Surface Area a) Consider deep reservoirs instead of wider reservoirs wherever possible 2. Mechanical Covers a) Permanent roofs b) Temporary roofs c) Floating roofs 3. Chemical Films Formation of monomolecular layes on water surfces by using 1. Cetyl alocohol 2. Stearyl alcohol Excellence and Service  CHRIST Deemed to be University REDUCTION OF EVAPORATION IN RESERVOIRS CHEMICAL FILMS Desirable features of chemical films ✓ The film should be strong and flexible to withstand wave action ✓ The film should form a homogeneous film if there is any puncture due to rain drops/ birds/ insects ✓ It should be pervious to oxygen and carbon dioxide ✓ It should be colourless, odourless and non toxic Excellence and Service  CHRIST Deemed to be University EVAPOTRANSPIRATION (ET) Evapotranspiration (ET) = Evaporation from soil and water + transpiration from plant leaves Excellence and Service  CHRIST Deemed to be University EVAPOTRANSPIRATION (ET) All the processes by which liquid water at/near land surface becomes atmospheric water: Evaporation + Transpiration Evaporation is the transfer of H2O from liquid to vapor phase. It may occur from water bodies, soil, and plant intercepted water. Transpiration is the evaporation occurring from plant leaves through stomatal openings (plant mediated evaporation). Excellence and Service  CHRIST Deemed to be University EVAPOTRANSPIRATION (ET) E and T are combined due to difficulties in separating the two Evaporation from: ✓ Water surfaces is relatively simple process to describe. ✓ Soil evaporation is more difficult. ✓ Transpiration from plants is even more complex. ✓ Fortunately the complex process of perspiration by mammals are not quantitatively important. 57% of all land “P” evaporates and oceans evaporates 112% of directly received “P” Excellence and Service  CHRIST Deemed to be University VARIOUS TERMS -ET Field Capacity: It is the maximum capacity of the soil to retain water against the force of gravity Permanent Wilting Point: It is the moisture content of the soil at which no moisture is available to sustain plants. Available Water: Difference Between Field capacity and Permanent Wilting Point Potential Evapotranspiration (PET): ET is called as PET, If sufficient moisture is available to meet the needs of vegetation growth throughout the crop period covering entire area. PET is influenced by Climatic Factors only. Actual Evapotranspiration (AET): The real ET occurring in a specific situation is called AET. Excellence and Service  CHRIST Deemed to be University ✓ The potential evapotranspiration (PET) is defined as the evapotranspiration that would result when there is always an adequate water supply available to a fully vegetated surface. ✓ This term implies an ideal water supply to the plants. In case water supply to the plant is less than PET, the deficient would be drawn from the soil moisture storage. ✓ With further moisture deficiency, the actual evapotranspiration (AET) will become less than PET until the Wilting Point is reached, and when the Evapotranspiration stops. Excellence and Service  CHRIST Deemed to be University POTENTIAL and ACTUAL ET PET is the ET when there is ample amount of water. Consider the analogy of supply vs. demand: PET = demand and AET = supply. Often we cannot meet the demand: PET ≥ AET. AET:PET is low in arid areas, whereas AET~PET in humid areas Excellence and Service  CHRIST Deemed to be University FACTORS AFFECTING EVAPOTRANSPIRATION 1. WEATHER 2. CROP CHARACTERISTICS 3. MANAGEMENT 4. ENVIRONMENTAL CONDITIONS Excellence and Service  CHRIST Deemed to be University FACTORS AFFECTING EVAPOTRANSPIRATION 1. WEATHER ✓ SOLAR RADIATION ✓ AIR TEMPERATURE ✓ RELATIVE HUMIDITY ✓ WIND SPEED Excellence and Service  CHRIST Deemed to be University FACTORS AFFECTING EVAPOTRANSPIRATION 2. CROP CHARACTERISTICS ✓ CROP TYPE AND VARIETY Height Roughness Stomatal Control Reflectivity Ground cover Rooting characteristics ✓ STAGE OF DEVELOPMENT Excellence and Service  CHRIST Deemed to be University FACTORS AFFECTING EVAPOTRANSPIRATION 3. MANAGEMENT ✓ IRRIGATION METHOD ✓ IRRIGATION MANAGEMENT ✓ CULTIVATION PRACTICES ✓ FERTILITY MANAGEMENT ✓ DISEASE AND PEST CONTROL Excellence and Service  CHRIST Deemed to be University FACTORS AFFECTING EVAPOTRANSPIRATION 4. ENVIRONMENTAL CONDITIONS ✓ SOIL TYPE, TEXTURE, WATER-HOLDING CAPACITY ✓ SOIL SALINITY ✓ SOIL DEPTH AND LAYERING ✓ POOR SOIL FERTILITY ✓ EXPOSURE/SHELTERING Excellence and Service  CHRIST Deemed to be University METHODS OF ESTIMATING ET DIRECT MEASUREMENT EMPIRICAL, SEMI-EMPIRICAL, AND PHYSICALLY - Based equations Using Climate And Weather Data Excellence and Service  CHRIST Deemed to be University DIRECT MEASUREMENT OF Evapotranspiration LYSIMETRY SOIL WATER DEPLETION ENERGY BALANCE AND MICRO-METEOROLOGICAL METHODS: Research applications only ✓ Mass Transfer / Bowen ratio o Vertical Gradients of Air Temp and Water Vapour Blaney-Criddle method ✓ Eddy Correlation o Gradients of Wind Speed and Water Vapour Penman’s method Excellence and Service  CHRIST Deemed to be University LYSIMETRY Excellence and Service  CHRIST Deemed to be University LYSIMETRY It is special water-tight tank containing a block of soil and set in a field of growing plants. Dimensions of the tank are 0.6 - 3.3 m Diameter with 1.8 - 3.3 m Depth It is buried to the Level of Natural Soil Same plants are grown in the tank w.r.t surrounding field ET is estimated in terms of amount of water required to maintain constant moisture condition; volumetrically or gravimetrically. Excellence and Service  CHRIST Deemed to be University LYSIMETRY Lysimeters should be designed to accurately reproduce The Soil Condition Moisture Content Type and Size of Vegetation of the Surrounding Area Lysimeter studies are Time Consuming Expensive Lysimeter studies are suitable only for experimental studies. Excellence and Service  CHRIST Deemed to be University METHODS OF ESTIMATING ET PENMAN’S EQUATION BLANEY-CRIDDLE FORMULA Excellence and Service  CHRIST Deemed to be University PENMAN’S EQUATION 𝐴𝐻𝑛 + 𝐸𝑎 𝛾 𝑃𝐸𝑇 = 𝐴+𝛾 PET: Daily Potential ET in mm/day A: Slope of ew v/s Temperature Curve at Mean Air Temperature, in mm of Hg/ °C ew: Saturation Vapour Pressure Hn: Net Radiation in mm of Evaporable Water per day Ea: Parameter including Wind Velocity and Saturation Deficit 𝛾: Psychrometric constant = 0.49 mm of Hg/ °C Excellence and Service  CHRIST Deemed to be University PENMAN’S EQUATION 𝐴𝐻𝑛 + 𝐸𝑎 𝛾 𝑃𝐸𝑇 = 𝐴+𝛾 𝐻𝑛 = 𝐻𝑎 + 𝐻𝑒 + 𝐻𝑔 + 𝐻𝑠 + 𝐻𝑖 𝑛 𝑛 𝐻𝑛 = 𝐻𝑎 1 − 𝑟 𝑎+𝑏 − 𝜎𝑇𝑎 4 0.56 − 0.092 𝑒𝑎 0.10 + 0.90 𝑁 𝑁 𝑢2 𝐸𝑎 = 0.35 1 + 𝑒𝑤 − 𝑒𝑎 160 Excellence and Service  CHRIST Deemed to be University PENMAN’S EQUATION 𝐴𝐻𝑛 + 𝐸𝑎 𝛾 𝑃𝐸𝑇 = 𝐴+𝛾 𝑛 𝑛 𝐻𝑛 = 𝐻𝑎 1 − 𝑟 𝑎 + 𝑏 − 𝜎𝑇𝑎 4 0.56 − 0.092 𝑒𝑎 0.10 + 0.90 𝑁 𝑁 𝑢2 𝐸𝑎 = 0.35 1 + 𝑒𝑤 − 𝑒𝑎 160 For the computation of PET ✓ n: actual duration of bright sunshine in hrs ✓ ea: actual mean vapour pressure in the air in mm of Hg ✓ u2: mean wind speed at 2m above ground in km/day ✓ mean air temperature, Ta: 273+°C in kelvin ✓ Nature of surface, r: Reflection Coefficient These can be obtained from actual observations or from meteorological data Excellence and Service  CHRIST Deemed to be University BLANEY-CRIDDLE FORMULA 𝐸𝑇 = 2.54 𝐾𝐹 𝑃ℎ 𝑇ഥ𝑓 F=෍ 100 ET: PET in a Crop Season in cm K: Empirical Coefficient; based on Type and Stage of Crop F: Sum of monthly Consumptive Use factors for the period Ph: Monthly percent of Annual Day-Time in hrs 𝑇ഥ𝑓 : Mean Monthly Temperature in °F Excellence and Service  CHRIST Deemed to be University INFILTRATION The process of water entering the soil at the ground surface after overcoming resistance to flow is called infiltration Excellence and Service  CHRIST Deemed to be University INFILTRATION: Moisture Zones in Soil Excellence and Service  CHRIST Deemed to be University INFILTRATION: Process Maximum rate, at which ground can absorb water - Infiltration Capacity Fixed Volume of water it can hold - Field Capacity Excellence and Service  CHRIST Deemed to be University INFILTRATION: Process Infiltrated water may contribute to Groundwater Discharge in addition to increasing the Soil Moisture Excellence and Service  CHRIST Deemed to be University INFILTRATION: Terms Infiltration Capacity, fp: It is the maximum rate at which a soil in any given condition is capable of absorbing water Infiltration Rate, f: It is actually the prevailing rate at which the water is entering the given soil at any given instant of time Both are expressed in cm/hr 𝑓 = 𝑓𝑝 𝑤ℎ𝑒𝑛 𝑖 ≥ 𝑓𝑝 𝑓 = 𝑖 𝑤ℎ𝑒𝑛 𝑖 < 𝑓𝑝 Where, i is the Intensity of Rainfall Excellence and Service  CHRIST Deemed to be University FACTORS INFLUENCING INFILTRATION CAPACITY Initial moisture content Condition of the soil surface Hydraulic conductivity of the soil profile Texture Porosity Degree of swelling of soil colloids Organic matter Vegetative cover Duration of irrigation or rainfall Viscosity of water ✓ Characteristic of Soil ✓ Surface Condition ✓ Fluid Characteristics Excellence and Service  CHRIST Deemed to be University FACTORS INFLUENCING INFILTRATION CAPACITY ✓ Characteristic of Soil The type of soil, (sand, silt or clay), its texture, structure, permeability and drainage are the important characteristics. A loose, permeable, sandy soil will have a larger infiltration capacity than a tight, clayey soil. Dry Soil can absorb more water than Wet Soil. The land use has a significant influence on infiltration rate. Excellence and Service  CHRIST Deemed to be University FACTORS INFLUENCING INFILTRATION CAPACITY ✓ Surface Condition At the soil surface, the impact of raindrops causes the fines in the soils to be displaced and these in turn can clog the pore spaces in the upper layers. Thus a surface covered by grass and other vegetation which can reduce this process has a pronounced influence on the value of fp Excellence and Service  CHRIST Deemed to be University FACTORS INFLUENCING INFILTRATION CAPACITY ✓ Fluid Characteristics Water infiltrating into the soil will have many impurities, both in solution and in suspension. The turbidity of the water, especially the clay and colloid content is an important factor as such suspended particles block the fine pores in the soil and reduce its infiltration capacity. The temperature of the water is a factor in the sense that it affects the viscosity of the water which in turn affects the infiltration rate. Contamination of the water by dissolved salts can affect the soil structure and in turn affect the infiltration rate. Excellence and Service  CHRIST Deemed to be University MEASUREMENT OF INFILTRATION Infiltration characteristics of a soil is estimated by; Flooding Type Infiltrometers Simple (Tube Type) Infiltrometer Double Ring Infiltrometer Measurement of subsidence of free water in a large basin or pond Rainfall Simulator Hydrograph Analysis Excellence and Service  CHRIST Deemed to be University DOUBLE RING INFILTROMETER Excellence and Service  CHRIST Deemed to be University DOUBLE RING INFILTROMETER Excellence and Service  CHRIST Deemed to be University DOUBLE RING INFILTROMETER: Principle Excellence and Service  CHRIST Deemed to be University Double Ring Infiltrometer: Infiltration Rate v/s Time ✓ Experiment is carried out until a constant rate of Infiltration is obtained. Excellence and Service  CHRIST Deemed to be University Double Ring Infiltrometer: Disadvantages ✓ Determine Infiltration characteristics at a spot only – require large number of experiments to obtain representative characteristics for an entire watershed. ✓ Raindrop effect is not simulated ✓ Driving of the rings disturbs the soil structure ✓ Results of the infiltrometers depend to some extent on their size with large meters giving less rates than the smaller ones – border effect Excellence and Service  CHRIST Deemed to be University HORTON INFILTRATION EQUATION 𝑓 = 𝑓𝑐 + 𝑓0 − 𝑓𝑐 𝑒 −𝐾𝑡 f = Infiltration rate at any time, t t = time from beginning of Rainfall fc = Infiltration rate after it reaches a constant value fo = Infiltration rate at start K = a constant Excellence and Service  CHRIST Deemed to be University HORTON INFILTRATION EQUATION 𝑓 = 𝑓𝑐 + 𝑓0 − 𝑓𝑐 𝑒 −𝐾𝑡 ln 𝑓 − 𝑓𝑐 = ln 𝑓0 − 𝑓𝑐 − 𝐾𝑡 Plot ln(𝑓−𝑓𝑐 ) against t and fit the best straight line through the plotted points. ln(𝑓0−𝑓𝑐 ) is the Intercept K is the Slope of the straight line Excellence and Service  CHRIST Deemed to be University HORTON INFILTRATION EQUATION 𝑓 = 𝑓𝑐 + 𝑓0 − 𝑓𝑐 𝑒 −𝐾𝑡 ln 𝑓 − 𝑓𝑐 = ln 𝑓0 − 𝑓𝑐 − 𝐾𝑡 Excellence and Service  CHRIST Deemed to be University HORTON INFILTRATION EQUATION Q. The infiltration capacities of an area at different intervals of time are indicated in the table below. Find an equation for the infiltration capacity in the exponential form Time t (hrs) 0 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 Infiltration Capacity f (cm/hr) 10.4 5.6 3.2 2.1 1.5 1.2 1.1 1 1 Excellence and Service  CHRIST Deemed to be University HORTON INFILTRATION EQUATION 𝑓 = 𝑓𝑐 + 𝑓0 − 𝑓𝑐 𝑒 −𝐾𝑡 f = Infiltration rate at any time, t t = time from beginning of Rainfall fc = Infiltration rate after it reaches a constant value fo = Infiltration rate at start K = a constant Excellence and Service  CHRIST Deemed to be University RAINFALL HYETOGRAPH Excellence and Service  CHRIST Deemed to be University INFILTRATION INDICES Excellence and Service  CHRIST Deemed to be University INFILTRATION INDICES It is convenient to use a constant value of infiltration rate – for estimating flood The defined average infiltration rate is called as Infiltration Index Two Indices are commonly used: φ - Index W - Index Excellence and Service  CHRIST Deemed to be University φ - INDEX Excellence and Service  CHRIST Deemed to be University φ - INDEX The Φ-index is the average rainfall above which the rainfall volume is equal to the runoff volume It is average infiltration rate during the period of rainfall excess Φ-index is derived from the rainfall hyetograph with the knowledge of the resulting runoff volume. The initial loss is also considered as part of infiltration If i < Φ-index then f=i and if i > Φ-index then f=Φ-index i is rainfall intensity and f is infiltration rate Excellence and Service  CHRIST Deemed to be University φ - INDEX Rainfall Excess Rainfall excess is the rainfall contribution to runoff and the period during which such a rainfall takes place is called period of rainfall excess In relation to runoff and flood studies, it is called as effective rainfall. Excellence and Service  CHRIST Deemed to be University φ – INDEX CALCULATION Given: Rainfall Hyetograph and Direct Runoff Use same unit: mm or cm for rainfall and runoff Represent as Incremental Rainfall if Cumulative Rainfall is given 1. Consider whole duration of Rainfall as Effective, te 𝑇𝑜𝑡𝑎𝑙 𝑅𝑎𝑖𝑛𝑓𝑎𝑙𝑙 −𝐷𝑖𝑟𝑒𝑐𝑡 𝑅𝑢𝑛𝑜𝑓𝑓 First Trial: 𝜑 = ൗ𝑡𝑒 2. Compute Rainfall Excess for each Rainfall Pulse Find Total Rainfall Excess 𝑅𝑎𝑖𝑛𝑓𝑎𝑙𝑙 𝐸𝑥𝑐𝑒𝑠𝑠 = 𝑃 − 𝜑 ∆𝑡; for i > Φ; else 0 ∆t is interval of rainfall data 3. Compare Total Rainfall Excess with Direct Runoff. If Re ≠ R, take another te 𝑇𝑜𝑡𝑎𝑙 𝑅𝑎𝑖𝑛𝑓𝑎𝑙𝑙 −𝐷𝑖𝑟𝑒𝑐𝑡 𝑅𝑢𝑛𝑜𝑓𝑓−𝐼𝑛𝑒𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 𝑅𝑎𝑖𝑛𝑓𝑎𝑙𝑙 Second Trial: 𝜑 = ൗ𝑡𝑒 4. Repeat until Re =R Excellence and Service  CHRIST Deemed to be University φ – INDEX Numerical Q1. A storm with 10.0 cm precipitation produce a direct runoff of 5.8 cm given the time distribution of the storm as below, estimate the φ - INDEX of the storm? Time from start (h) 1 2 3 4 5 6 7 8 Incremental rainfall 0.4 0.9 1.5 2.3 1.8 1.6 1.0 0.5 in each hour (cm) Excellence and Service  CHRIST Deemed to be University φ – INDEX Numerical Time from start (h) 1 2 3 4 5 6 7 8 Incremental rainfall 0.4 0.9 1.5 2.3 1.8 1.6 1.0 0.5 in each hour (cm) 1. Consider whole duration of Rainfall as Effective, te 𝑇𝑜𝑡𝑎𝑙 𝑅𝑎𝑖𝑛𝑓𝑎𝑙𝑙 −𝐷𝑖𝑟𝑒𝑐𝑡 𝑅𝑢𝑛𝑜𝑓𝑓 First Trial: 𝜑 = ൗ𝑡𝑒 2. Compute Rainfall Excess for each Rainfall Pulse Find Total Rainfall Excess 𝑅𝑎𝑖𝑛𝑓𝑎𝑙𝑙 𝐸𝑥𝑐𝑒𝑠𝑠 = 𝑃 − 𝜑 ∆𝑡; rainfall intensity > Φ; else 0 ∆t is interval of rainfall data 3. Compare Total Rainfall Excess with Direct Runoff. If Re ≠ R, take another te 𝑇𝑜𝑡𝑎𝑙 𝑅𝑎𝑖𝑛𝑓𝑎𝑙𝑙 −𝐷𝑖𝑟𝑒𝑐𝑡 𝑅𝑢𝑛𝑜𝑓𝑓−𝐼𝑛𝑒𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 𝑅𝑎𝑖𝑛𝑓𝑎𝑙𝑙 Second Trial: 𝜑 = ൗ𝑡𝑒 4. Repeat until Re =R Excellence and Service  CHRIST Deemed to be University W - INDEX The W index is a redefined version of Φ index It exclude the depression storage and interception from the total losses It is the average infiltration rate during the time rainfall intensity exceeds the capacity rate 𝑊 = 𝑇𝑜𝑡𝑎𝑙 𝑅𝑎𝑖𝑛𝑓𝑎𝑙𝑙 𝑃 −𝐷𝑖𝑟𝑒𝑐𝑡 𝑅𝑢𝑛𝑜𝑓𝑓 𝑅 −𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝐿𝑜𝑠𝑠 (𝐼𝑎) ൗ𝑡𝑒 W-index < Φ-index Excellence and Service  CHRIST Deemed to be University RUNOFF WATER BALANCE P = Q + ET + (S/t) Excellence and Service  CHRIST Deemed to be University RUNOFF WATER BALANCE ET = evapotranspiration (cm/h) ET P f = infiltration rate (cm/h) P = precipitation (cm) Si Qi = interflow (cm) f Qs ground Sr Qg = groundwater runoff (cm) Qs = surface runoff (cm) qg Qi qd = geological water loss (cm) Ss qg = groundwater recharge (cm) qd Sd = surface detention (cm) Sg Qg Si = interception (cm) Sg = groundwater storage (cm) Sr = surface retention (cm) Ss = soil moisture storage (cm) t = time interval (h) Excellence and Service  CHRIST Deemed to be University RUNOFF WATER BALANCE ET P Si f Qs Surface Runoff ground Sr qg Qi Interflow Ss Sg qd Qg Groundwater flow Excellence and Service  CHRIST Deemed to be University RUNOFF WATER BALANCE ET P Si f Qs Qs = P – Si – Sr - ft ground Sr Ss qg Qi Qi = (f-ET) t – Ss – qg Sg qd Qg Qg = qg – Sg - qd Excellence and Service  CHRIST Deemed to be University RUNOFF WATER BALANCE ET = evapotranspiration (cm/h) f = infiltration rate (cm/h) P = precipitation (cm) Qi = interflow (cm) Qg = groundwater runoff (cm) Qs = P – Si – Sr - ft Qs = surface runoff (cm) qd = geological water loss (cm) Qi = (f-ET) t – Ss – qg qg = groundwater recharge (cm) Sd = surface detention (cm) Si = interception (cm) Qg = qg – Sg - qd Sg = groundwater storage (cm) Sr = surface retention (cm) Ss = soil moisture storage (cm) t = time interval (h) Excellence and Service  CHRIST Deemed to be University Excellence and Service